The invention relates generally to a high pressure seal and specifically to a seal for use in pressure vessels as well as manufacturing and research equipment operating at pressures as great as 100,000 p.s.i. and temperatures several hundreds of degrees above and below ambient.
The behavior of various substances at extremely high pressures and temperatures approaching absolute zero as well as several hundreds of degrees above ambient is becoming the subject of expanding basic research and experimentation. The prospect of a technology based upon the characteristics and performance of substances subjected to such extremes has prompted both the government to fund basic research and private interests to attempt to utilize such knowledge.
Examination of the chemical and physical properties of substances subjected to high pressure requires sophisticated equipment, such as high pressure compressors, pressure vessels and pressure vessel seals which are capable of maintaining their mechanical and structural integrity at pressures as high as 50,000 p.s.i. or even 100,000 p.s.i.
The design criteria for pressure vessels have been well established. A typical pressure vessel may have an internal volume of from 300 to 500 cubic centimeters (18 to 30 cubic inches) while large pressure vessels may contain several thousands of times these internal volumes. Such a pressure vessel is typically of monoblock construction, fabricated of annealed 304 stainless steel, and includes a closure and threaded gland nut also of 304 stainless steel, as well as a seal disposed between the closure and pressure vessel. The operating pressure limit of such vessels is generally in the vicinity of 40,000 p.s.i. Larger pressure vessels for both cryogenic or high temperature applications are preferably fabricated by forging from 9% nickel steel. Such vessels exhibit yield strengths of approximately 110,000 p.s.i. and increase operating pressure limits proportionately.
Likewise, the design goals for pressure vessel seals have been well established. For example, P. W. Bridgman in The Physics of High Pressure (1949) described various high pressure seals and postulated his principle of unsupported area. Subsequently, Chua, Terry and Ruoff disclosed a multi-part, small (approximately one inch outside diameter, one-half inch inside diameter) seal incorporating indium and brass components which utilized Bridgman's unsupported area principle.
High pressure research is often directed to the frontiers of knowledge and technology. Consequently, pressure vessel technology has occasionally failed to keep pace with the demands placed upon it by increasingly sophisticated high pressure research. Such was the situation as research into the properties of substances subjected to both high pressure and low temperature began. Large pressure vessel seals which could withstand both pressure as high as 4080 bars (60,000 p.s.i.) and temperature as low as that of liquid helium (4.2 Kelvin) do not exist.
As the sophistication of manufacturing processes has increased, similar situations, i.e., disparities between seal capabilities and seal requirements, have become manifest in this field also. For example, the cracking of oil tar sands to produce petroleum distillates is performed at extremely high pressure and elevated temperature. Existing seals have limited reliability and pressure cycle life at these conditions. At the present time, there are no wholly satisfactory seals for this application.
The difficulties of operation at such extremes in temperature and pressure are manifold. First of all, the pressure vessel and thus the seal is subjected to a dimensional change as the vessel is pressurized. When a typical 304 stainless steel pressure vessel having a cavity diameter of 7.612 centimeters (3.0 inches) is subjected to a pressure of 1530 bars (22,500 p.s.i.) the width of the gap between the top closure and walls of the pressure vessel occupied by the seal increases by approximately 0.0137 millimeters (0.00054 inches) at ambient temperature. At cryogenic temperatures, such gap width increase is reduced. For example, at the temperature of liquid nitrogen (77.3.degree. Kelvin) the width of the seal increases by 0.0096 millimeters (0.00038 inches) and at the temperature of liquid helium (4.2.degree. Kelvin) the seal width increase is reduced to 0.0074 millimeters (0.0003 inches).
Although such reduced dimensional changes at cryogenic temperatures might initially suggest that this difficulty is of reduced significance at reduced temperatures, precisely the reverse is true. First of all, since the majority of all components exhibiting any sealing capability are fabricated of materials other than 304 stainless steel, the seal components will exhibit a different thermal shrinkage due to their unique thermal coefficient of expansion. Thus, the temperature change from ambient to near absolute zero will produce a disparate and additional dimensional change between the seal components and the pressure vessel in addition to that dimensional change caused by the pressurization of the vessel. For example, the thermal coefficient of expansion of silicon rubber, a typical seal material, is approximately ten times larger than the thermal coefficient of expansion of 304 stainless steel. If the coefficients of expansion for both materials are assumed to remain constant from room temperature to the temperature of liquid nitrogen, a corresponding differential between the pressure vessel diameter and the seal diameter results.
Secondly, since seal components are generally fabricated of one of a variety of resilient materials such as silicon rubber, Buna-N, fluorocarbon rubber or Teflon, as the temperature is reduced the seal material becomes brittle and inflexible, looses compressibility and is apt to fracture. Inasmuch as it is deemed apparent why such resilient materials are especially suited for utilization as seal components, it is likewise deemed apparent why the loss of such resiliency and compressibility renders them unsuitable for low temperature seal applications.
In addition to this temperature related problem, conventional resilient seal components exhibit a pressure related difficulty. During pressurization, gas within the pressure vessel diffuses into the seal material. Upon depressurization, this diffused gas cannot escape and blisters and occasionally fractures the entire cross section of the seal. This problem is aggravated by increasing the cycle rate at which the pressurization-depressurization sequence occurs. Although this difficulty becomes apparent only at the conclusion of a pressurization trial, it renders the affected seal components unusable and necessitates disassembly of the pressure vessel seal and replacement of such damaged components. In addition to the obvious cost of replacement of a seal component and the time consumed to replace it, such single cycle reliablility is impractical in any application requiring repeated pressurization and depressurization without disassembly.
It is therefore clear that conventional pressure vessel seals which may function acceptably at ambient temperature are increasingly subject to thermal contraction mismatch and loss of resilience as operating temperatures lower and approach absolute zero. These difficulties have their analogs as the operating temperature of the pressure vessel is elevated above ambient. The problem of mismatch between the thermal coefficients of expansion of the pressure vessel and the seal material is, of course, analogous but for the fact that the seal material now expands more than the pressure vessel and may be subject to undue deformation. Similarly, at elevated temperatures lack of seal resilience is no longer a problem but excessive resilience is. Most of the resilient materials delineated above become soft and subject to uncontrolled distortion and extrusion. If the temperature is sufficiently high, the seal material will extrude due to the high pressure and become permanently deformed. While this problem is, of course, a matter of degree, these materials generally function unsuitably at all but relatively low pressures and temperatures near ambient.
Numerous solutions to the difficulties discussed above have been suggested. Single O-ring seals of silicon rubber and Buna-N were found to be subject to blisters and fractures due to diffused gas. Creavey seals consisting of Teflon (fluorocarbon resin) tubular torus having a stainless steel resilient spring inside with a Viton (fluoroelastomer) core at its center have been found to repeatedly fail due to distortion at the junction of the outer Teflon jacket. Various combinations of metallic and resilient O-ring seals have also been attempted but with decreasing success as operating temperatures approach absolute zero. Attempts to increase the radial compression O-ring seals by the use of oversize cross section and oversize mean diameter O-rings has also met with limited success. Such oversize components lead to problems during installation which cause twisting and distortion of the O-rings, excessive squeeze and excessive compressive loadings. Inoperable seals and/or reduced life are generally the result of such attempts at oversizing. The use of lubricants to facilitate the assembly process has been found to cause extensive O-ring rupture.
At least one-structural solution to the dimensional change problem has been suggested. It is the process of pre-pressurization and it entails pressurizing the vessel prior to lowering (or raising) its temperature. The previously delineated seal types and materials exhibit somewhat improved performance when pre-pressurized but this process severely inhibits operational flexibility in that the seals will not withstand pressure cycling at the reduced (or elevated) temperature. This can be a serious drawback since it, in effect, requires that the pressure vessel be warmed (or cooled) to ambient at the beginning of every pressure cycle to perform the pre-pressurization step--a process that takes several hours. In other words, a seal which will function satisfactorily only if pre-pressurized, will not function properly in an application where it is necessary or desirable to perform repeated pressurization-depressurization cycles without returning to ambient temperature.